Herpes simplex viruses (HSV) encode a surprisingly high number of functions designed to thwart host responses to infection. Included in the list of function blocked by HSV are programmed cell death induced both by specific functions of viral gene products or by exogenous agents (Fas ligand, tumor necrosis factor, thermal or osmotic shock, etc). Specifically, in the absence of one of several gene viral products, HSV infection results in induction of apoptosis at multiple steps during its replicative cycle. Both the induction and pathway of programmed cell death may be cell type-dependent. The proposal described in this application is based on three key observations: (i) Virus mutants lacking glycoprotein D (gD) induce apoptosis. Apoptosis induced by gD about mutants is blocked independently by expression of gD or gJ delivered in trans bybaculovirus vectors. (ii) A mutant, d120 lacking the gene encoding ICP4, the major regulatory protein, induces apoptosis in all cell lines tested. Studies in progress indicate that apoptosis is induced by ICPO, a promiscuous transactivator overexpressed in d120-infected cells. ICPO made in d120 mutant-infected cells differs from wild type ICP0 in several properties. Apoptosis induced by the d120 mutant is blocked in cells overexpressing Bcl-2 and also independently by viral protein kinase Us3. (iii) Cells contain a complex of DFF40 (a DNase responsible for fragmentation of cellular DNA) and DFF45 (its inhibitor). In cells induced to apoptosis by exogenous agents, cytochrome C is released, caspases are activated, DFF45 is cleaved, and cellular DNA is degraded. In cells infected with wild-type virus and induced by osmotic shock, caspases are activated but DFF45 is protected from cleavage. The Us3 protein kinase appears to be involved in blocking cleavage of DFF45. The objectives of the studies proposed in this application are (i) to determine the mechanisms by which gD- viruses induce apoptosis and the means by which gD or gJ block it; (ii) to defme the mechanism by which ICP0 induces apoptosis and the effective target of Us3 in blocking it (iii) to elucidate the mechanism by which HSV blocks the cleavage of DFF45 and precludes fragmentation of cellular DNA.